1. Field of the Invention
The present invention pertains to a head holding apparatus that is used in devices that test one or both of the head and disk of a hard disk drives, and relates to a device having a means for controlling the amount of head displacement per unit of time with exposure to heat, so that fluctuations in test results are controlled.
2. Discussion of the Background Art
Hard disk drives (HDDs) are widely used for information recording media for recording large volumes of digital data, beginning with electronic computers.
HDDs generally comprise a substrate, one or more magnetic disks, which is a non-magnetic disk material covered with a magnetic thin film, a rotating spindle that is anchored to the substrate and rotates the magnetic disk at high speed, a slider with a head, which is the recording element and reproducing element, at its surface, head gimbal assemblies (HGAs) with the sliders at its end, a suspension arm that supports the HGA, and a rotary actuator that is anchored to the substrate and drives the arm. Furthermore, one HDD has several magnetic disks and heads, depending on the recording capacity of the HDD.
The working principle of the HDD is as follows: The center of a magnetic disk is held by a spindle that rotates at high speed from 4,000 to 15,000 rotations per minute. The slider is guided by an arm that is driven by a rotary actuator and moved so that the trace between the outer periphery and the innermost periphery of the magnetic disk forms an arc. Moreover, when information is being recorded or retrieved, the slider that is above the magnetic disk maintains a tilted posture so that a wedge-shaped space is formed with the magnetic disk as the slider floats a very small distance above the magnetic disk in the air current that is produced on the surface of the magnetic disk that is rotating at a high speed. Once the slider has been positioned at a predetermined position on the magnetic disk by the rotary actuator, the magnetic disk is magnetized and information is recorded by the recording element attached to the slider. Moreover, the magnetic field from the magnetic disk is detected and information is retrieved by the reproducing element attached to the same slider.
Furthermore, recording and retrieving of information are performed in the memory area that has been made by physically subdividing the magnetic disk recording surface. For instance, reading and writing are performed on an circular memory area called a track having a predetermined width that has been made along the concentric circumference of the magnetic disk.
The HDD accumulates information and therefore, each part comprising the HDD undergoes rigorous testing because there must be complete reliability during recording and retrieving of information. A head testing device that records or retrieves information on a magnetic disk for testing and evaluates the performance and properties of the head is used to test heads.
An oblique view of a conventional head testing device 10 is shown in FIG. 1A, and a side view of the same head testing device 10 is shown in FIG. 1B. Head testing device 10 in FIGS. 1A and 1B consists of reference table 11, cassette 30 that holds head 20 at the end, carriage 12 that holds cassette 30, piezo stage 13 that fine-positions carriage 12 horizontally with respect to reference table 11, head loading mechanism (HLM) 14 that positions piezo stage 13 perpendicularly with respect to reference table 11, stage 15 anchored above reference table 11 that coarse-positions HLM 14 horizontally with respect to reference table 11, and disk rotating device drive 50 anchored above reference table 11 that holds the center of magnetic disk 40 with rotating shaft 51 so that magnetic disk 40 is horizontal with respect to reference table 11 and magnetic disk 40 is rotated around its axis using motor 52.
Cassette 30 has head holding part 31 that holds head 20, connector 32, support 33 that is connected to carriage 12 by connector 32 and supports head holding part 31, and amplifier 35 that is connected to head 20 via signal line 34 and processes electrical signals transmitted to and received from head 20. Cassette 30 can be disconnected from the head testing device as needed because it has connector 32 and therefore, the heads that are the subject of tests performed by the head testing device can be replaced as needed when heads are loaded on each cassette.
Moreover, although not illustrated, in addition to the above-mentioned structural elements, head testing device 10 has external arithmetic and control unit M and input-output unit P. Arithmetic and control unit M controls stage 15, HLM 14, piezo stage 13 and disk rotating device drive 50 based on commands that are input by the user through input-output unit P, and further, is connected to amplifier 35 and transmits and receives electrical signals to and from amplifier 35 in order to process and analyze the electrical signals that are transmitted and received, etc. In addition, the analysis results and operating status, etc., are further input to input-output unit P and communicated to the user as needed.
A summary of the effects of the above-mentioned structure is as follows: Piezo positioner or stage 13 is coarse-positioned by stage 15 and then carriage 12 is fine-positioned by piezo stage 13. Head 20 is positioned at a predetermined position above magnetic disk 40 by these positioning operations. Furthermore, head 20 is moved up and down above magnetic disk 40 by HLM 14 and floats above the surface of magnetic disk 40 or rests above the surface of magnetic disk 40. Head 20 generates a magnetic field when it floats above the surface of magnetic disk 40 and writes information on magnetic disk 40 or detects a magnetic field and reads information from the magnetic disk.
The following are items evaluated by a head testing device: the track average amplitude (TAA), which is the average amplitude of retrieving signals that are output from the head; the track profile (TP) representing the distribution of TAAs relative to displacement from the track center line (TCL) within a track; the overwrite (OW), which is represented by the attenuation factor of the lowest frequency signals (LF signals hereafter) when the highest frequency signals (HF signals hereafter) are overwritten on LF recorded on a magnetic disk; the bit error rate (BER); etc. The intensity of the magnetic field generated by the magnetic information that has been written on the magnetic disk changes with the position of the head within a track. Consequently, the head positioning accuracy (positioning accuracy hereafter) in the direction of track width above the magnetic disk has a strong effect on the measurement accuracy of the test items when evaluating the above- mentioned test items. Particularly high head-positioning accuracy is needed in BER tests, etc., because determination time is long in comparison to the other test items.
Nevertheless, it has become difficult to achieve the head-positioning accuracy that is now required with the progress that has been made in HDD technology. The reason for this is that signals that are transmitted to and received from the head have become faster as a result of the recent increase in the data transmission speed of the HDD. The amplifier of the head testing device must be placed near the head in order to control the attenuation of faint signals from the head, and there are many cases where the amplifier is loaded on the cassette. However, the amplifier begins to generate heat at the same time when the measurements are started. Therefore, the dimensions of the cassette change as it expands when exposed to the heat generated by the amplifier. As a result, the head slips (drifts hereafter) from the predetermined position within the determination track and there is a marked compromise of measurement reliability.
A cross section of cassette 30 is shown in FIG. 2. It is an example of the effect of drift. Cassette 30 has head holding part 31 that holds head 20, connector 32, support 33, and amplifier 35 connected to head 20 via signal line 34, and further has anchoring pin 36 that by all appearances protrudes to the side where the cassette is connected to carriage 12. Anchoring pin 36 is pushed into hole 37 in carriage 12 in order to secure the reference point for the entire cassette 30 when cassette 30 is connected to carriage 12. For convenience, connector 32 is not illustrated. When the effective length relating to the positioning of support 33, which supports and positions the head, that is, the distance in the direction of length of support 33 from the center of anchoring pin 36 to the point of application of head 20, is 50 mm and support 33 is made from corrosion-resistant aluminum (linear expansion coefficient of 23.4 ppm), thermal expansion of support 33 will occur with a change in temperature of 1xc2x0 C., causing it to drift 1,200 nm. A substantial amount of drift of 590 nm occurs when the skew angle, which is the angle formed by the track tangent and the head, becomes 30xc2x0. A drift of 590 nm corresponds to displacement by 2 track widths or more when a head of 100 kTPI, or track interval of 250 nm, is tested, and causes the head to be completely off the measurement track.
Constructing a cassette using a material with a small thermal expansion coefficient, such as Invar, etc., has been considered as a method of controlling drift, but materials with a small linear expansion coefficient are primarily ferromagnetic and are inappropriate for head testing devices that use magnetic fields because they have an effect on the determinations. Moreover, controlling the effect of drift by positioning with a piezo stage when the cassette is in a thermally stable state, that is, when the amount of drift has become constant, has also been considered, but there is a problem in that stand-by time until the cassette reaches a thermally stable state impacts production cost.
Therefore, there is a demand for a device with which the amount of displacement of the head per of unit time is controlled, even during the transient period until the cassette reaches a thermally stable state, that is, the period when there is continuous marked thermal expansion of the cassette, so that the positioning accuracy of the head required by the cassette is not compromised, even in cases when the head testing device begins testing the head immediately after the cassette has been connected to the testing device.
The purpose of the present invention is to solve the above-mentioned problems of prior art, its object being to control the amount of displacement of the head per unit of time, which is caused by thermal expansion of the support, and thereby control fluctuations in the measurement results that are obtained when testing one or both of the head and the disk by supporting the support of the head by means of a heat-compensating member in the cassette that holds the head.
Moreover, another object is to control the size of the cassette by using a structure where two or more parts with different linear expansion coefficients are layered in the heat-compensating member.
In short, the first subject of the invention is a head holding apparatus that is used in order to hold a head or the assembly on which said head is loaded in testing devices that test one or both of said head and recording medium or disk, characterized in that it comprises an anchoring means that anchors the position of the above-mentioned head holding apparatus and a support that supports the above-mentioned recording element at a predetermined position with the above-mentioned anchoring means as the criterion, and in that the above-mentioned support is also supported by a heat-compensating member arranged parallel to the above-mentioned support and when the above-mentioned support expands with exposure to heat, the heat-compensating member expands by the same amount as the above-mentioned support to control the amount of displacement of the head per unit of time with exposure to heat.
The second subject of the invention is characterized in that one or both of the above-mentioned support and the above-mentioned heat-compensating member of the first subject of the invention has a structure wherein the friction that is produced between the above-mentioned support and the above-mentioned heat-compensating member is reduced at that place where the above-mentioned support and the above-mentioned heat-compensating member oppose one another so that the above-mentioned support and the above-mentioned heat-compensating member will expand smoothly when exposed to heat.
The third subject of the invention is characterized in that there is a damping member placed between the above-mentioned support and the above-mentioned heat-compensating member at that place where the above-mentioned support and the above-mentioned heat-compensating member oppose one another in the second subject of the invention so that the resonance of the above-mentioned support is controlled.
Furthermore, the fourth subject of the invention is characterized in that there is a means for reducing the temperature difference between said support and said heat-compensating member in the first, second or third subject of the invention.
In addition, the fifth subject of the invention is characterized in that the above-mentioned heat-compensating member in the first, second, third or fourth subject of the invention has a structure wherein two or more members with different linear expansion coefficients are alternately layered.